Load-indicating means for a nut and bolt assembly

ABSTRACT

A load-indicating nut in which the nut body has at one end a radially extending flange and wherein the cross section of the flange taken in a radial plane is such that there is provided an internal surface to the annular flange which takes the form of an annular groove of generally V-section with the apex thereof directed radially outwardly and the one leg of this V-section which is joined to the body being constructed and arranged so that when the axial tensile load in the bolt shank reaches or exceeds a predetermined minimum value, plastic deformation of the metal of the flange takes place in shear in this one leg and the body moves in a direction towards the abutment against which it is being tightened so that there occurs a measurable reduction in the overall axial dimension of the nut.

United States Patent 1191 cm et al.

[ Feh.5,1974

[ LOAD-INDICATING MEANS FOR A NUT AND BOLT ASSEMBLY [75 Inventors: PeterJohn Gill, Wolverhampton;

Dennis Joseph ODonnell, Walsall, both of England [73] Assignee: GKNScrews & Fasteners Limited,

Smethwick, Warley, England [22] Filed: May 22, 1972 [21] Appl. No.:255,679

188,224 11/1966 U.S.S'.R 151/38 Primary Examiner-Marion Parsons, Jr.

Attorney, Agent, or Firm-Merriam, Marshall, Shapiro & Klose [5 7]ABSTRACT A load-indicating nut in which the nut body has at one end aradially extending flange and wherein the cross section of the flangetaken in a radial plane is such that there is provided an internalsurface to the annular flange which takes the form of an annular grooveof generally V-section with -the apex thereof directed radiallyoutwardly and the one leg of this V-section which is joined to the bodybeing constructed and arranged so that when the axial tensile load inthe bolt shank reaches or exceeds a predetermined minimum value, plasticdeformation of the metal of the flange takes place in shear in this oneleg and the body moves in a direction towards the abutment against whichit is being tightened so that there occurs a measurable reduction in theoverall axial dimension of the nut.

5 Claims, 4 Drawing Figures PATENTEDFEB slam 3.789.726

' SHEET 1 OF 2 BACKGROUND OF'THE INVENTION 1. Field of the Invention Itis well known and accepted that to achieve maximum efficiency in a nutand bolt fastening it is necessary to tighten the nut and bolt assemblyto such an extent that the tension generated in the bolt shank reaches,and preferably exceeds, a certain predetermined minimum. This isparticularly important in certain applications such as high strengthbolted connections and also in nut and bolt fastenings used in aircraftwhere it is very essential to ensure that nut and bolt assemblies arecorrectly tightened and that there shall be no slackening of theconnection under workingconditions where the nut and bolt assembly issubjected to vibrations and other fluctuating stresses.

The present invention relates to means which will provide a detectableindication (e.g., visual and/or measurable so that it can be known thatthe nut and bolt assembly has been correctly tightened to such an extentas to ensure that the desired minimum tension in the bolt shank has beenreached or exceeded.

The indicating means to which the present invention relates may beincorporated in the .body of the nut or the head of the bolt.Hereinafter the expression body (except where otherwise qualified, suchas by referenceto the accompanying drawings) refers to the body of a nutor the head of a bolt. The invention is concerned with indicatingmeanswherein the body is formed-in such a manner that, when used in anut and bolt assembly which is tightened, a portion or portions of thebody is plastically deformed when the tension'in the shank of the boltreaches or exceeds a predetermined minimum value, the deformation beingsuch as to give a detectable indication (e.g., visual and/or measurable)in such a way that it can be known that the desired minimum tension inthe bolt shank has been reached or exceeded.

2. Description of the Prior Art A prior proposal in this field for aload-indicating nut and load-indicating bolt has used the device ofdetecting and measuring the change in dimension of a gap which existsinitially between a portion or portions of the body and an abutmentagainst which the body istightened, the change in dimension of such gaptaking 1 placewhen the desired minimum tension is reached in the boltshank and the formation of the body around the gap being such that whenthis desired minimum tension is reached portions of the body adjacentthe gap undergo plastic deformation and the body moves axially to reducethe dimensions ofthe gap. In such prior proposal gaps were provided onthe underside of a nut body or on the underside of the head of a bolt.

In one particular version of such prior proposal a nut suitable forapplicationin aircraft nut and bolt fastenings has a base flange of agenerally triangular configuration providing three bearing pads toengage an abutment at three apices and along the three sides of theflange there were provided recesses in the underside of the flangewhich, when the flange was in engagement with an abutment in theassembly; defined gaps between the flange and the abutment and such gapswere used to give the measurable indication referred toabove.

Certain drawbacks exist with the load-indicating nut disclosed in theaforesaid specification, one of which is 1 due to the fact that thepresence of the recesses in the three sides of the flange means thatthere is not a continuous surface-bearing area of flange to engage theabutment and aircraft manufacturers and users of such nuts desire tohave a bearing surface on the underside of the nut which isuninterrupted so as to give full face contact with the abutment over thewhole of the surface area of the flange. Further, the nature of theflange of the nut in the prior proposal being, as it was, interrupted onthree sides by the aforesaid recessesintroduced difficulties inmanufacture of the nut by cold forming techniques as it was founddifficult in practice to ensure that metal would flow to completely fillall the confines of the die and form a perfect flange on each occasion.

The-manner of detecting and checking that such nuts of the priorproposal had been correctly tightened in use was by means of a feelergauge introduced between the surface of the abutment and the underneathof the flange of the nut in the gap along a side.

In another prior proposal a load-indicating washer has projections onone face formed by slitting and pressing out portions of the metal ofthe washer, such projections being intended to become almost flattenedback into the plane of the washer when the desired tension has beenachieved in the bolt shank.

SUMMARY OF THE INVENTION The object of the invention is to provide animproved form of load-indicating nut or bolt which avoids the abovementioned drawbacks and enables a more accurate method of checking to beused to ensure that a nut and bolt assembly has been tightened to thedesired extent.

Accordingly, for use in a nut and bolt assembly the invention provides aload-indicating nut or bolt wherein the body includes a radiallyoutwardly extending flange, the underside of which provides acontinuous, uninterrupted planar abutment-engaging surface of annularform to provide uninterrupted engagement over an annular area, with anabutment in the assembly, the partof the flange which connects with thebody being inclined .to the axis-of the body and being constructed andarranged so that, when a predetermined tension is reached in the boltshank during tightening of the assembly, such inclined part undergoes aplastic deformation resulting in a reduction of the overall axialdimension of the body, measured in the direction of the axis of theassembly.

The cross section of the flange, taken in a radial plane, may be suchthat there is provided an internal surface to the annular flange whichtakes the form of an annular groove of generally V-section with the apexthereof directed radially outwardly and the one leg of the V-sectionwhich is joined to the body is constructed and arranged so that when theaxial-tensile load inthe bolt shank reaches or exceeds a predeterminedminicompanying drawings which illustrate a form of loadindicating nutembodying the features of the invention and suitable for use in theaircraft industry.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENT With reference to FIG. 1, the nut has a body, indicated at10, and at one end thereof a radially outwardly extending flange,generally indicated at 11, the body being provided in the normal waywith an internal thread 12 and on its exterior with wrench-engagingsurfaces 13. Any convenient form of wrench-engaging surfaces may beprovided on the exterior of the body 10, the form shown in the drawingbeing that which is generally known as a 12 point nut.

Furthermore, where the nut is used in aircraft, the body is preferablyprovided in the manner indicated in the prior specification hereinbeforereferred to with a further externally plain extension at its one end 14which extension may be provided with a known form of locking or stifffeature such as by inwards deformation of the wall of the plain portionto produce a portion of thread at this outer end of the nut which exertsan increased frictional grip upon the threads of a mating bolt to resistany tendency for the nut to unscrew when used in applications where itis subjected to vibration.

The flange 11 is of annular form and, as seen in the right-hand side ofFIG. 1, has a cross section taken in a radial plane which provides aninternal surface of the flange in the form of an annular groove 15 whichis of V-cross section with the apex 16 of the groove presented in theradially outwards direction. The groove V- section advisably has itsapex directed radially outwardly so that the apex defines a circle witha diameter at least equal to the diameter of the body. The height of thegroove as measured parallel to the axis of the body furthermoreadvisably is substantially equal to the height of the flange as measuredparallel to the axis of the body. The one leg 17 of the flange which isconnected with the body 10 is inclined to the axis of the body and theother leg 19 of the annular flange gradually increases in radiallymeasured thickness from its join with the leg 17 until it reaches theunderside of the flange where there is provided the uninterruptedannular surface 18 which is the abutment-engaging surface hereinbeforereferred to. The under surface 18 is complete and uninterrupted andprovides an annular under surface which can be dimensioned to providethe desired bearing surface area for the particular size of nut.

The leg 17 of the annular flange provides a portion which undergoesplastic deformation as hereinafter described and, as can be seen in FIG..1, the thickness of this leg 17 measured radially decreasesprogressively from its connection with the body 10 to its connectionwith the other leg 19, or in other words this leg tapers 4 v inthickness. The amount of tapering is slight and has been exaggerated inthe drawing in order to make it more clear. This dimensioning andarrangement of the annular flange ensures that the connection of theflange with the body 10 is the strongest part of the leg 17 of theflange and also ensures that, when under load, there is no danger ofshearing taking place along the cylindrical surface indicated by thegenerator Y-Y, but ensures that when the predetermined minimum tensionin the bolt shank is reached or exceeded the result is that the leg 17undergoes plastic deformation in or around the general region of theconical surface indicated by the generator XX. Further, it will beobserved that the leg 19 has a radially measured thickness whichincreases progressively from the point where it connects with the leg 17down to the underside surface 18 so that the leg 19 is stronger than theleg 17 and this undersurface 18 provides the complete uninterruptedannular surface which is the bearing surface for engagement with anabutment in the complete assembly of nut and bolt.

Generally, therefore, the design of the annular flange l 1 is such as toprovide for the plastic deformation taking place in the leg 17 but alsoso as to avoid any large concentration of stress at a particular pointwhich might lead to shearing at some part of the flange. The design isalso such as to minimise dilation of the body 10 when the predeterminedtension is achieved and exceeded in the bolt shank. In addition to theplastic deformation which takes place in the leg 17 in the region abovereferred to there can be some generaly movement of theflange 11 as awhole so as to avoid concentration of stress in any particular place andthere can be some actual dilation of the flange 11 which dilation mayalso be measured as an indication of the point at which the desiredminimum tensile stress in the bolt shank has been achieved.

When the nut is tightened upon a bolt and after the surface 18 of theflange 11 has engaged-with the abutment surface 18in a nut and boltassembly, further tightening causes increase of tension in the boltshank and thus increase in the stresses in the whole nut and boltassembly until the stress system in the metal of the leg 17 of theflange reaches or exceeds the elastic limit whereupon plasticdeformation takesplace in the region of the imaginary conical surface XXas hereinbefore described and there occurs a reduction in the overallaxial height of the nut. The stress at the aforesaid region is mainlyshear but more strictly speaking it is a complicated stress system ofcombined shear, compression and bending.

The graph of FIG. 4 is a typical curve of axial load (tension) in thebolt shank plotted against deformation (change in height of nut) in anut and bolt assembly;

As can be seen from this graph there occurs plastic flow of the metal inthe section 17 at substantially constant load and thus if tightening iscarried on until any point in the range marked by the section betweenthe dotted lines on the graph, the amount of plastic deformationindicated thereby will ensure that the minimum tensile load required inthe bolt shank has been reached and exceeded. Thus, there is aconsiderable amount of latitude available for the operator to ensurethat he has tightened the nut down to achieve and exceed the desiredtensile load.

As there is also a certain relationship between applied torque andtensile load generated in the bolt 7 shank, it is preferable, ifpossible, to use a known type of torque-indicating or torque-limitingwrench for tightening the nut so that the operator has a visualindication or other form of indication from the wrench that he hasmostprobably achieved the desired tensile load, but as the relationshipbetween torque and tensile load can be affected by other factors thefinal checking of the tightening nut can be done with the kind of deviceillustrated in FIG. 3'to ensure an accurate measurement of the reductionin overall height of the nut which is a measurement of the amount ofplastic distortion which has taken place in the section 17 of theannular flange.

Instead of using a torque-indicating or torquelimiting wrench theexperience of the operator may be relied upon and other techniques maybe used, such as that whereby after engagement has been achieved betweenthe surface 18 and the abutment surface of the assembly the operatorgives the nut a further predetermined amount of turn which is calculatedto achieve the desired amount of plastic deformation in the section 17and which can thereafter be checked by the apparatus shown in FIG. 3. 4

The apparatus shown in FIG. 3 is a modification of a generally knownform of gauge which has a clocktype dial 21 and a feeler stem 22, theaxial extension of which is measured accurately and indicated by theneedle upon the'dial 21.

To adapt such form of instrument to the present application the end ofthe stem 22 is provided with a shoe 23 having a lower annular edge 24which is adapted to engage upon a surface, at the upper end of the nut.Fixed to a. boss 25-on the exterior of the dial 21 there is an outercylindrical shoe 26 which extends downwardlyand has a lower annularsurface 27 adapted to engage against the abutment surface in the nut andbolt assembly so that the dial 21 gives an accurate measurement of thedistance between the surfaces 24 and 27 of the shoes 23 and 26. A spring28 acts upon the shoe 23 via the lower end of the stem 22 to maintainthis in firm engagement with the upper face of the nut.

Thus, after the nut has been tightened down by the operator to what isthought to be the required extent the so tightened nut can be checked byan inspector using the device of FIG. 3 and an accurate indicationobtained asto whether the desired amount of deformation has beenobtained in the section 17, as indicated by the reduction in overallheight of the nut, so as to ensure that the desired tension in the boltshank has been reached or exceeded.

The nut as above described can be conveniently manufactured by a coldforming process to produce the nut body and the wrench-engaging surfacesand an annular flange having initially a plain cylindrical internalsurface but having the frusto-conical surface 30 defining the outer faceof the section 17, and the fillet which joins theflange to the body 10.The blank so produced may then be held in the chuck of a lathe orsimilar machine and the groove cut by'applying a suitable cutting toolthrough thelopening in the lower face of the nut, the tool being appliedat the correct angle to obtain the V-section groove and, during suchcutting, accurate location of the nut body can be obtained by virtue ofthe frusto-conical surface 30 being accurately located in the chuck.This surface 30 provides a surface of reference for the accurate cuttingaction of the tool to provide the leg section 17 of the desired crosssectional area.

We claim:

1. For use in a nut and bolt assembly, a load indicating nut or boltcomprising a body including a flange having a single continuousuninterrupted planar abutment-engaging surface of annular forr'n capableof engaging an abutment over an annular area, the flange having aninternal annular groove of generally V- shaped cross-section taken in aradial plane, the V- section having its apex directed radiallyoutwardly, the flange being divided by the V-shaped groove into a pairof legs, one extending from the body to the apex of the V and theother-extending from the apex of the V to the abutment-engaging surface,the shape of the flange and groove being such as to define, in the firstmentioned leg, a yield portion of restricted thickness inclined withrespect to the axis of the body and disposed so that, when apredetermined tension is reached in the bolt shank during tightening ofthe assembly the yield portion undergoes plastic deformation in shear inan annular region which is defined by the rotation of an imaginarygenerator about the axis of the body, said generator being inclinedoutwardly away from said axis considered in the direction away from theabutment engaging surface, and the plastic deformation taking place insuch a sense that it results in a reduction of the overall axialdimension of the body.

2. A nut or bolt according to claim 1 wherein the thickness of said oneleg of the V-section flange decreases progressively from its connectionwith the body to the apex of the V.

3. A nut or bolt according to claim 2 wherein said other leg of theV-section flange gradually increases in thickness, measured radially,from the apex of the V- shaped groove to the abutment engaging surfaceof the flange.

4. For use in a bolt and nut assembly, a load indicating nut or boltcomprising a body and a flange having a single continuous uninterruptedplanar abutment engaging surface of annular form capable ,of engaging anabutment over an annular area, the flange having an internal annulargroove of generally V-shaped crosssection taken in a radial plane, theV-section having its apex directed radially outwardly and the apexdefining a circle of diameter at least equal to the diameter of thebody. I

5. A nut or bolt according to claim 4 wherein the height of the grooveas measured parallel to the axis of the body is substantially equal tothe height of the flange as measured parallel to the axis of the body.

1. For use in a nut and bolt assembly, a load indicating nut or boltcomprising a body including a flange having a single continuousuninterrupted planar abutment-engaging surface of annular form capableof engaging an abutment over an annular area, the flange having aninternal annular groove of generally V-shaped cross-section taken in aradial plane, the V-section having its apex directed radially outwardly,the flange being divided by the V-shaped groove into a pair of legs, oneextending from the body to the apex of the V and the other extendingfrom the apex of the V to the abutment-engaging surface, the shape ofthe flange and groove being such as to define, in the first mentionedleg, a yield portion of restricted thickness inclined with respect tothe axis of the body and disposed so that, when a predetermined tensionis reached in the bolt shank during tightening of the assembly the yieldportion undergoes plastic deformation in shear in an annular regionwhich is defined by the rotation of an imaginary generator about theaxis of the body, said generator being inclined outwardly away from saidaxis considered in the direction away from the abutment engagingsurface, and the plastic deformation taking place in such a sense thatit results in a reduction of the overall axial dimension of the body. 2.A nut or bolt according to claim 1 wherein the thickness of said one legof the V-section flange decreases progressively from its connection withthe body to the apex of the V.
 3. A nut or bolt according to claim 2wherein said other leg of the V-section flange gradually increases inthickness, measured radially, from the apex of the V-shaped groove tothe abutment engaging surface of the flange.
 4. For use in a bolt andnut assembly, a load indicating nut or bolt comprising a body and aflange having a single continuous uninterrupted planar abutment engagingsurface of annular form capable of engaging an abutment over an annulararea, the flange having an internal annular groove of generally V-shapedcross-section taken in a radial plane, the V-section having its apexdirected radially outwardly and the apex defining a circle of diameterat least equal to the diameter of the body.
 5. A nut or bolt accordingto claim 4 wherein the height of the groove as measured parallel to theaxis of the body is substantially equal to the height of the flange asmeasured parallel to the axis of the body.